This invention relates generally to the field of integrated circuit design and fabrication. Specifically, the invention relates to ion-assisted oxidation methods and the structures formed thereby.
Silicon oxide is used in integrated circuit (IC) and semiconductor fabrication, especially metal-oxide-semiconductor (MOS) fabrication, due to its excellent dielectric properties for insulation and field enhancement. See, for example, U.S. Pat. Nos. 4,776,925 and 5,521,126, the disclosures of which are incorporated herein by reference. Silicon oxide, both doped and undoped, is especially useful for fabrication of transistors. A transistor is typically fabricated by growing a gate structure containing a gate dielectric and a gate conductor on the surface of a silicon substrate, followed by forming source and drain regions in portions of the substrate flanking the gate structure. Unfortunately, the gate dielectric will be etched partially during the dry etch of the gate conductor. A thinner gate dielectric along the gate conductor edge will degrade the gate dielectric intensity and increase the gate-induced drain leakage. In order to eliminate these problems, the gate dielectric needs to be re-grown.
A high-temperature (greater than about 700° C.) oxidation is often used in the IC industry to re-grow the gate dielectric. High-temperature oxidation, however, can cause problems such as changes in impurity profiles, non-uniform oxide thicknesses along the side wall of the gate conductor, and defect formation in the underlying substrate. When certain metals, such as tungsten, are used as the gate conductor, a high-temperature oxidation process can form materials, such as tungsten oxide, that are volatile and can cause product yield loss.
Different oxidation techniques, such as plasma oxidation and anodization, have been proposed to reduce the temperature necessary to re-grow gate dielectrics. Most of these low- temperature techniques, however, fail to produce the high-quality gate dielectric necessary to maintain good gate dielectric integrity and low leakage.